This invention is directed to a field emission liquid metal ion source which has a needle which is heated and is coated with liquid metal to emit ions.
The liquid metal ion source is a relatively new device. Several liquid metal ion sources have been made in the recent past, and those designed by Roy Clampitt and L. W. Swanson are of particular interest. The Roy Clampitt source is disclosed in United Kingdom Pat. No. 1,442,998 discussed below and in several papers: R. Clampitt and D. K. Jefferies, "Miniature Ion Sources for Analytical Instruments", Nuclear Instruments and Methods, 149, (1978), pp. 739-742; and R. Clampitt and D. K. Jefferies, "Molten Metal Field Ion Sources", Institute for Physics Conference, Series No. 38 (1978), Chapter 1, pp. 12-17. Swanson's work was presented as "Emission Characteristics of a Liquid Gallium Ion Source" at SEM/1979, Apr. 16-20, 1979 at Washington, D. C. by L. W. Swanson, G. A. Schwind and A. E. Bell.
Most liquid metal ion sources to date have employed metals or alloys possessing low melting points. Gallium, indium, bismuth and cesium are examples of metals which have been used in such sources. Nearly all prior liquid metal ion sources are characterized by a solid or tubular needle protruding from a closed reservoir, and the reservoir is used for holding the liquid metal. Liquifying the metal has usually been done by heating the reservoir indirectly, either radiantly or conductively. United Kingdom Pat. No. 1,442,998 shows a plurality of adjacent field emission points to which liquid ion source material is fed by capillary action.
There are considerable disadvantages to these prior art structures, particularly when they are employed with metals or alloys which have melting points higher than those listed above. These disadvantages include the fact that they are time consuming to manufacture and as a result are expensive to the user. Furthermore, the reservoirs are impossible to refill and thus a new ion source is required whenever the reservoir is exhausted. The needle is inefficiently heated and the desired temperature gradient is hard to achieve. The direct heating of the needle tip by backstreaming secondary electrons is hard to detect.
The gas field ionization "hairpin" ion source when used with a liquid metal, was initially thought to offer solutions to some of those problems. The heart of the hairpin device is a U-shaped heater wire with a needle welded to the apex of the U. The heater wire was used to clean the attached needle by heating it to cause outgasing. The device was originally designed for gas field emission. L. W. Swanson first used the device as a liquid metal source, by applying liquid metal directly to the needle, but a number of drawbacks were found. The hairpin device is difficult to make, due to the necessary welding of the needle to the U-shaped heater wire. Since the needle is welded to the wire, the device lacks versatility because the amount of needle protrusion beyond the heater wire is fixed. Since the needle is mounted on the heater wire, and the heater wire is employed for structural support of the needle, the ion source lacks stability in the direction perpendicular to the plane of the U-shaped heater wire when the heater wire is heated. As a result there is need for an improved field emission liquid metal ion source.